Small-scale biogas production in the province of Pampanga, Philippines

Abstract: The purpose of this study was to contribute to sustainable development in the Pampanga province, Philippines. The Philippines are facing several major environmental problems; pig production represents a major contributing factor to pollution and eutrophication of water bodies in the country. At present there is no requirement for purification of wastewater from backyard (small-scale) farms. With the help of anaerobic digestion the manure could be used to produce biogas. The study’s objective was to determine an anaerobic digester design and substrate composition for small-scale biogas production in backyard farms (20 pigs). To achieve this objective, several goals were defined; (1) determine the best substrate composition for biogas production, through the use of lab experiments, (2) determine a design best suited for small-scale use, (3) test the substrate and design in-field, (4) determine the impact of different inoculum, (5) determine a small-scale design based on local conditions such as finance, building materials, climate etc., (6) evaluate the digestates’ properties as a bio-fertilizer and (7) compare the biogas production with an existing biogas plant in Pampanga. Four combinations of food waste and pig manure were tested at Karlstad University’s biogas lab. The best composition had VS-ratio 1:2 for pig manure to food waste, and produced 111.1 NmL CH4/g VSadded. The chosen design was a floating drum digester. It was tested together with the substrate and two inoculums (cow manure and digestate from an existing biogas plant) in four pilot plants during 32 days. The plants showed symptoms of inhibition in biogas production and the pH dropped to toxic levels (below pH 6). At the end of the study, the plants had similar cumulative biogas production. The mean production was 1764 NmL and the average methane content was 33.6%. The plants with inoculum from an existing biogas plant showed signs of recovering from the low pH. The plants with cow manure as inoculum showed increasing signs of inhibition. The mean phosphorus content of the digestate was 1.3% TS and nitrogen content was 6% TS. The carbon to nitrogen ratio was on average 8:1, which had a positive impact on nitrogen mineralization in the soil (if used as bio-fertilizer). The digestate had high levels of VFA, which reflected improper substrate composition. The large fraction of food waste was the likely cause of inhibition; easily digestible carbohydrates lead to the pH drop and high levels of VFA. Based on the field study and literature, another substrate composition was recommended for the final design. The suggested substrate had VS-ratio 2:1 for pig manure to food waste. A variant of the floating drum digester was recommended, where several digesters could be connected in series and the number of digesters could be adapted to the number of pigs. The digester volume was 6.2 m3; the expected biogas production was 2.6 m3 with 70% methane content. This facility would result in a digestate containing 0.2 kg phosphorus and 0.7 kg nitrogen per day. Comparisons between the existing plants’ biogas production and the pilot plants could not be done due to the fact that the plant lacked methods for measuring their biogas production and methane content. However, the study’s overall findings make a strong argument for the use of biogas in backyard farms, for mitigating many of the Philippines’ water quality issues arising from pig production.